This invention relates to the use of a variable displacement hydraulic pump in a load sensitive hydraulic system. The hydraulic system will utilize a variable displacement hydraulic pump to supply fluid to a priority dependent work circuit and to at least one other secondary hydraulic work circuit. Priority means are incorporated to assure that the priority dependent work circuit will be supplied with required fluid at the necessary volume and pressure before the secondary work circuit requirement is fulfilled.
This invention is an improvement over prior developments in load sensitive hydraulic circuitry making these prior embodiments more adaptable to applications having utility in many fields. A recent patent (U.S. Pat. No. 4,034,563 to H. R. Orth) provided the basic system of load sensitive hydraulic circuitry of which this system is an improvement to. Also a prior patent (U.S. Pat. No. 3,750,405 to R. J. Lech, et al, Aug. 7, 1973), assigned to the same assignee as the instant invention and herein incorporated by reference, discloses the type of hydraulic system that would be subject to improvement through the utilization of the instant invention. Also, an article appearing in the September, 1975 issue of "Automotive Engineering" magazine entitled "Load-Sensitive Hydrostatic Steering--A New Approach" based on S.A.E. paper 750806 by J. L. Rau shows the state of the art using the apparatus set forth in the above mentioned Lech patent.
The instant invention is an improvement to the state of the art, including the recent patent to H. R. Orth, yielding a result that makes load sensitive steering, as well as other priority dependent systems useful, simple, and workable in actual applications by overcoming deficiencies in the contemporary art.
One feature not alleviated in the the Lech patent or the periodical article is a tendency of the primary or priority dependent circuit to recoil or kickback when a secondary work circuit is opened while high pressure is needed in the primary or priority dependent circuit. This has been one factor deleterious to the wide spread use of load sensitive priority circuits. This problem has been remedied through the provisions of the device set forth in the mentioned Orth patent.
The instant invention is a refinement of the Orth patent that has been engineered to prevent the undesirable kickback features through the use of a simpler valve system not incorporating the isolator valve of the Orth patent.
A typical application of this simplified load sensitive system is in farm tractors where a single pump may be utilized to provide fluid to such devices as, but not limited to, a steering unit, a brake unit, a hydraulic hitch as well as secondary circuits for other hydraulic devices such as implement motors, fan drive motors and bucket actuating motors (cylinders), as well as other secondary hydraulic systems and apparatus. The vehicle control devices generally must have priority of hydraulic fluid allocation for safety reasons. Often the fluid power required by primary circuits is of variable flow rates at various pressures while still having priority over the secondary circuits.
The use of the variable displacement load sensitive system is desirable as this system provides fluid at demanded rates without the waste of power not needed by the operating hydraulic systems.